This invention relates to Josephson junction circuits and, in particular, to such circuits having enhanced current gain.
According to the original scheme of J. M. Rowell (U.S. Pat. No. 3,281,609), Josephson tunnel junctions having hysteretic I-V characteristics are used to perform logic functions with a fixed bias current I.sub.b by employing a magnetic field to reduce the critical current I.sub.J below I.sub.b. The resulting voltage can be used to generate current through a load, and the magnetic field generated by this load current can then be used to control Josephson junctions in subsequent logic stages.
Analysis of this technique for controlling Josephson junctions indicates that the magnetic field B required to reduce I.sub.J significantly must be large enough that the flux .phi. linking the junction is .phi.=k.phi..sub.o, where .phi..sub.o is the flux quantum and k is of the order of unity. Thus, a junction of length l and magnetic thickness (2.lambda.+d) requires EQU B=k.phi..sub.o /]l(2.lambda.+d)] (1)
where .lambda. is the London penetration depth and d is the thickness of the barrier layer. In addition, the magnetic field underneath a control line of width w carrying a current I.sub.c is EQU B=.mu..sub.o I.sub.c /W (2)
where .mu..sub.o is the magnetic permeability. Combining equations (1) and (2), we see that for a current in a control line to have an appreciable effect on I.sub.J it must be of order ##EQU1## For a typical value of 2.lambda.+d, say 1600 Angstroms, .phi..sub.o =2.07.times.10.sup.-15 Webers and .mu..sub.o =4.pi..times.10.sup.-7 henry/meter, equation (3) gives ##EQU2## So, for a square junction with a control line of the same width, a typical configuration, one must deal with critical currents of several mA. Note also that the junction bias currents themselves will create magnetic fields of the same order, leading to the conclusion that this logic design requires junctions with considerable self-field effects. For junctions which are not square, the current levels are proportionately smaller; but the same problem holds in that appreciable self-field effects are encountered. Self-field effects tend to cause undesirable resonance features at lower voltages and smaller ratios of the maximum-to-minimum I.sub.J in the presence of magnetic fields.
Much the same problem occurs with interferometers. In order to get enough gain, the critical currents and bias currents must be large enough to cause appreciable self-magnetic fields. In the case of interferometers, this requirement implies that the loop inductances L and critical currents I.sub.J must be such that LI.sub.J .gtorsim.k.phi..sub.o, where K is of the order of unity.
One approach to solving this problem is described in a modified goalpost (GP) circuit shown in FIG. 3 of my U.S. Pat. No. 4,051,393. There, a plurality 18 of branches containing junction-resistor combinations are connected in parallel with a GP in order to increase the fan-out current (col. 8, lines 29-45). However, this circuit operates in an avalanche mode with the parallel branch nearest GP 12 being first to switch to a high impedance state, and, conversely, with the farthest branch being last to switch. As a result, relatively long times are required to complete switching, thereby limiting the speed of the logic circuits.